Control over plasma asymmetry in a low-pressure capacitively coupled plasma
(CCP) discharges is vital for many plasma processing applications. In this
article, using the particle-in-cell simulation technique, we investigated the
asymmetry generation by a temporally asymmetric waveform (sawtooth-like) in
collisionless CCP discharge. A study by varying the number of harmonics (N)
contained in the sawtooth waveform is performed. The simulation resultspredict
a non-linear increase in the plasma density and ion flux with N i.e., it first
decreases, reaching a minimum value for a critical value of N, and then
increases almost linearly with afurther rise in N. The ionization asymmetry
increases with N, and higher harmonics on the instantaneous sheath position are
observed for higher values of N. These higher harmonics generate multiple
ionization beams that are generated near the expanding sheath edge and are
responsible for an enhanced plasma density for higher values of N. The ion
energy distribution function (IEDF) depicts a bi-modal shape for different
values of N. A strong DC self-bias is observed on the powered electrode, and
its value with respect to the plasma potential decreases with an increase in N
due to which corresponding ion energy on the powered electrode decreases. The
simulation results conclude that by changing the number of harmonics of a
sawtooth-like in collisionless CCP discharges, the ion flux asymmetry is not
generated, whereas sheath symmetry could be significantly affected and
therefore a systematic variation in the ion energy asymmetry is observed. Due
to an increase in the higher harmonic contents in the sawtooth waveform with N,
a transition from broad bi-modal to narrow-shaped IEDFs is found